A student in your class mentally calculates 8+9 by noting that 8 is "1 less than 9", and, since 2 x 9 = 18, then 8 + 9 must be "1 less than 18," or 17. The equations representing this method are as follows: 1.8+9 (9-1) +9 ii. (9-1)+9=9+(9-1) iii. 9+ (9-1) (9+9)-1 iv. (9+9)-1-18-1 V. 18-1 17 Which properties of addition is the student implicitly using?
